1. Field of the Invention
This invention relates to thermodynamically miscible polymer blends consisting of: (I) an aromatic polyetherimide (PEI); (II) an aromatic alkyl methacrylate polymer (PAAM) such as polybenzyl or polyphenyl ethyl methacrylate; and, optionally, (III) an aromatic polycarbonate (PC).
2. Discussion of the Background
The term "thermodynamically miscible" is used in the art to define a polymer blend that is mixed on the molecular level so as to form a single, homogeneous phase which exhibits only one glass transition. In contrast, the term "mechanically compatible" is taken to mean that mixing of the polymers is on a small scale but larger than the molecular level. Furthermore, "mechanically compatible" implies that the multiple phases exhibit good adhesion to one another so as to yield good mechanical properties. Although both thermodynamically miscible and mechanically compatible blends exhibit good mechanical properties, thermodynamically miscible blends will generally be stronger, and only thermodynamically miscible blends are transparent, owing to their single phase nature.
Blends of aromatic polyetherimides (PEI) and acrylate polymers are known. Aromatic polyetherimides are typically transparent, amorphous thermoplastics which display a good balance of properties; most notable are their good electrical properties, resistance to creep, high tensile strength and high heat deflection. Their good thermal stability, low mold shrinkage and resistance to solvents, acids and bases make them potentially attractive engineering resins. Nevertheless, most polyetherimides suffer from poor processability, low impact strength and high production cost. As a matter of fact, many polyetherimides are not used directly but instead are formed in situ from the more tractable polyamic acid precursors. Thus, their utility is limited to films and coatings in some cases. Acrylic polymers on the contrary display excellent workability and often excellent transparency. However, they suffer from poor dimensional stability and heat distortion.
PEI/polyacrylate blends can result in resins with comparatively more satisfactory properties with respect to the individual components. Unfortunately, many of these polymers blends are not miscible, nor compatible over most compositions. Blends of polyacrylates and polyetherimides displaying improved impact strength and processability over the polyetherimide alone have been disclosed in U.S. Pat. No. 4,395,518 but these blends are limited to less than 30% by weight of the acrylate polymer and they are not transparent or thermodynamically miscible. Similarly, EP Appl. 141,347 describes homogeneous blends of polyetherimides with acrylic elastomers that are clear and transparent only when the polyacrylate component is limited to less than 15 wt. %. High levels of such acrylate polymers in polyetherimides gives rise to poor processing characteristics and properties due to delamination of the part. Delamination is a direct result of the poor compatibility between the polymers, as is described in U.S. Pat. No. 4,673,708.
Blends of Ultem 1000 polyetherimide (General Electric) with polymethyl methacrylate or polyethyl methacrylate have been studied by the present inventor and found to exhibit two glass transitions when measured by differential scanning calorimetry (DSC), as expected (See Comparative Examples 1 and 2 below). Compression molded films of these blends are opaque and brittle, typical behavior of an incompatible mixture. However, if the methacrylate polymer contains aromatic alkyl ester groups, such as polybenzyl methacrylate, then thermodynamically miscible mixtures with PEI can be obtained. In addition, it has been discovered that polyaromatic alkyl methacrylates can be used to compatibilize PEI and aromatic polycarbonate resins.
Aromatic polycarbonates, particularly bisphenol A polycarbonate, also exhibit relatively good dimensional stability, transparency and stiffness as well as very good impact strength. Nevertheless, PC does have drawbacks such as poor scratch resistance, poor long term U.V. resistance and stress birefringence which have to be dealt with in demanding optical applications. Furthermore, typical polycarbonates, such as those made from bisphenol A, do not meet many local and federal requirements for flame retardance.
PEI/PC blend compositions result in materials with comparatively better properties with than those of the individual, single components. Blends of PEI and PC disclosed in U.S. Pat. No. 4,548,997 exhibit higher heat stability, higher heat distortion resistance, improved flexural strength and tensile strength over the PC component alone, as well as improved impact strength over the associated PEI component. In addition, the blends exhibit increased flame retardance as the level of PEI is increased in the mixtures. Unfortunately, the two materials are not miscible and the binary blend must be processed at high temperatures comparable to the processing of PEI alone. As a result of this high processing temperature, various undesirable reactions can occur including transpolymerization, oxidation, chain scission and other forms of degradation. Furthermore, the compatibility of the PEI and PC in U.S. Pat. No. 4,548,997 is also debated since it is stated that PEI cannot be blended, only mixed, with aromatic polycarbonates since two distinct glass transitions (Tg) are obtained unless the undesirable transpolymerization reaction occurs. Unfortunately, the resulting material formed by the transpolymerization reaction of PEI and PC has undesirable impact resistance and solvent resistance, according to EP 325,719 .
Therefore, the task existed of finding a PEI/PC blend which exhibits the following desirable characteristics: better average properties than those of the individual components; 2) lower processing temperature, especially in regard to PEI, so as to avoid undesirable side reactions; 3) absence of the transpolymerization side product which simulates miscibility but which does not possess the desirable properties; and 4) thermodynamic miscibility throughout the whole composition range so as to ensure transparency.
PEI/PC blends with more satisfactory properties over those of the individual components have been disclosed in the above-mentioned U.S. Pat. No. 4,548,997; however, such blends do not display lower processing temperatures over those of the single components nor are the blends monophasic with one Tg (thermodynamically miscible) except if the undesired transpolymerization product forms.
PEI/PC blends with lower processing temperatures over those of the single components have been disclosed in U.S. Pat. No. 4,673,708. This improvement in processing temperature has been achieved by means of the addition of a minor amount (less than 5 weight %) of an acrylic rubber. The declared intent of this patent, however, was to improve the impact strength of the PEI/PC blend. Furthermore, a single phase mixture was not obtained by the use of such an acrylic modifier. It is further stated in U.S. Pat. No. 4,673,708 that additional amounts of this acrylic rubber cause severe incompatibility among the components resulting in delamination of a molded part.
EP 325,719 discloses that the use of a phosphorous type stabilizer in a PEI/PC blend inhibits the undesirable transpolymerization side product but that the mixture remains two phase. Moreover, such blends neither display the required lower processing temperatures nor demonstrate transparency owing to the immiscibility of the components.
Accordingly, there is a need for transparent and easily processable polymer blends containing aromatic polyetherimides and polycarbonates. It has been found that aromatic alkyl methacrylate polymers have a high degree of compatibility with PEI over the entire compositional range, so as to overcome the deficiencies discussed hereinabove and provide single phase alloys with desired properties ranging from that of the pure polyetherimide to that of the polyacrylate. I have also discovered that PEI and polycarbonate will form a single-phase mixture in the presence of at least 20 wt. % of an aromatic alkyl methacrylate polymer, while as little as 10% PAAM can dissolve and plasticize PEI and PC simultaneously, resulting in a shift in Tg of both engineering resins and allowing a reduced processing temperature.